Mau Mau prisoners of the British were held in terrible conditions, say recent studies. Photograph: Terrence Spencer/Time & Life Pictures/Getty Image

The government is invoking an obscure legal principle to dismiss claims of torture and rape by the British colonial administration in Kenya, campaigners claimed.

The Foreign Office has said four elderly Kenyans alleging that they suffered serious physical and sexual abuse at the hands of the British during the Kenyan "emergency" of 1952 to 1960 should not be allowed to proceed with their claim because of the law of state succession.

The government argues it is "not liable for the acts and omissions of the Kenyan colonial administration", claiming the Kenyan government was now responsible for events that took place while Kenya was a British colony. But a cross-party group of MPs will this week publish an open letter demanding an apology and the creation of a welfare fund to help the alleged victims through old age.

Allegations that the British abused suspected Mau Mau fighters have continued since the Kenyan government lifted a 30-year ban on membership in 2003.

The organisation, which came into being to oppose colonial rule in Kenya, remains a sensitive issue because of the violence suffered by Kenyans. The British government recently acknowledged that suffering was experienced "on both sides" during the Mau Mau uprising in what experts said was the first recognition that the UK was also to blame.

A Foreign Office spokesman said the emergency period caused great pain on all sides, and marred progress towards independence.

But the government is refuting liability for the case, in which the claimants describe allegedly being castrated, sexually assaulted and beaten during their detention by the British and say they are still suffering consequences.

The case could open the way for up to 12,000 Kenyans to seek redress. It was filed at the high court last year. Daniel Leader, a lawyer at Leigh Day, representing the claimants, said: "One ... was castrated for supplying a cow to the Mau Mau."

"The nature and scale of this abuse was unparalleled in modern British colonial history. The claimants are among the poorest in Kenyan society, and they still live with injuries from that period.

"Historians have been through the public records, and the use of systematic violence was authorised at the highest level in London," Leaderhe said. "We have the documents to prove that."

But the government decision to have the case struck out on technical grounds of state succession – the principle that countries assume liability for their own affairs after independence – has infuriated human rights campaigners, who accuse the UK of shirking its responsibilities for rights abuses in former colonies.

The Foreign Office is believed to be arguing on a rule derived from a case over licences to fish for Patagonian toothfish in the South Georgia and South Sandwich islands, British overseas territories. "The FO is arguing that responsibility for acts by the colonial government passed to the independent government in 1963," Muthoni Wanyeki, executive director of Kenya Human Rights Commission, said.